Substituted Indazolobenzoxazines via One-Pot Intramolecular Bis-Heterocyclizations
J. D. Butler, D. M. Solano, L. I. Robins, M. J. Haddadin, M. J. Kurth*
University of California at Davis, USA and American University of Beirut, Lebanon
21. Februar 2008 (online)
Substituted indazolobenzoxazine B heterocycles have been synthesized from o-nitro derivative A by two sequential intramolecular heterocyclizations under basic conditions (KOH in alcoholic solvent) in one pot that presumably occurs via a nitroso imine intermediate. The intramolecular reaction is preferred to the competing intermolecular variant. In further reaction, the carboxylic acid at C9 position (indazole part of B) successfully underwent amidation with primary and secondary amines to give C in good yields. On the benzoxazine subunit of B, Sonogashira coupling was achieved successfully with terminal alkynes to give D, which upon 1,3-dipolar cycloaddition reactions with three different nitroalkanes gave C5-substituted isoxazole derivative E.